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The $(π^-,γγ)$ reaction in nuclei and the $σ$ meson in the medium

A theoretical analysis of the $(π^-,γγ)$ reaction in nuclei is made in order to find the viability of this reaction to test modifications of the $σ$ meson mass in nuclear matter. The $ππ$ correlation in the scalar-isoscalar channel in nuclear matter could, in principle, manifest itself in this reaction since it plays an important role in the $ππ\toγγ$ mechanism. But we conclude that this effect is hardly visible in this reaction due to the strong background of the pion-Bremsstrahlung terms. Only with some special cuts and for some polarization states are the effect visible at the cost of a strong reduction in the cross section.